Computer Assisted Tomography (CAT) represents the main research area of the Section on Neuroradiology. Our experience with one of the devices for this technique, the ACTA-Scanner, continues. We are acquiring direct clinical experience with another unit, the EMI-Scanner, which was installed in May of 1975 in the Clinical Center: more than 1,700 patients have been studied. A body EMI-Scanner and a second (experimental) head EMI-Scanner are being installed. Preliminary feasibility tests to build a totally new type of CAT device, which will use protons rather than x-rays, are well advanced. A number of mathematical and physical projects have been completed or are being carried out: review of the reconstruction algorithms; effect of beam hardening on the quality of the scan; signal vs. noise ratio; computer assisted subtraction. Clinical studies include CAT research on: demyelinating diseases, degenerative and atrophic processes, postradiation necrosis, spinal cord diseases, oculo-orbital lesions.